1. Field of the Invention
This invention relates to a diffractive optical element, and particularly to a diffractive optical element for use with light of a plurality of wavelengths or bands, and an optical system and an optical apparatus using the same.
2. Related Background Art
In a refracting optical system, use has heretofore been made of a method of decreasing chromatic aberration by a combination of glass materials differing in dispersion. In contrast, a method of decreasing chromatic aberration by providing a diffractive optical element having the diffracting action on a lens surface or in a portion of an optical system is disclosed in such literature as SPIE, Vol. 1354, International Lens Design Conference (1990), Japanese Patent Application Laid-Open No. 4-213421 (corresponding U.S. Pat. No. 5,044,706), Japanese Patent Application Laid-Open No. 6-324262 (corresponding U.S. Pat. No. 5,790,321), etc.
Further, a diffractive optical element as shown in FIG. 12 of the accompanying drawings is proposed as a diffractive optical element for use in an optical system. This diffractive optical element is of a construction in which a first diffraction optical part 2 having a phase-type diffraction grating 6 formed on a substrate 4 and a second diffraction optical part 3 having a phase-type diffraction grating 7 formed on a substrate 5 are disposed in proximity to each other with an air layer 8 interposed therebetween. The diffraction grating 6 and the diffraction grating 7 are formed of materials differing in dispersion value. This diffractive optical element is characterized in that it also is a diffractive optical element through the whole layer.
The diffraction efficiency of this diffractive optical element is shown in FIG. 13 of the accompanying drawings. In a laminated-type diffraction grating having a plurality of layers of diffraction gratings shown in FIG. 12, high diffraction efficiency can be maintained in the entire visible region as shown in FIG. 13 by suitably setting the material forming the diffraction gratings of respective layers and the grating height. Now, in FIG. 13, diffraction efficiency is shown as the rate of the design order diffracted light to the whole transmitted beam. Actually, however, Fresnel reflection occurs in the boundary between the air and the surface of the diffraction grating and therefore, in an interface, diffraction efficiency is reduced by several % in the entire wavelength region.
Accordingly, taking the Fresnel reflection on the interfaces between the air layer and the respective diffraction gratings into account, the diffraction efficiency of the whole is reduced by nearly 10% in the construction of FIG. 12.
In order to regulate this reduction in diffraction efficiency, it would come to mind to provide anti-reflection coating in the boundary between the air layer and the surface of the grating.
However, when anti-reflection coating is vapor-deposited on the boundary between the air layer and the surface of the grating, the following various problems arise.
Firstly, from the ease of preparation of the grating shape, a polymeric resin material is often used for a diffraction grating, and there is a problem attributable thereto. That is, it is that the polymeric resin material is weak to heat and therefore the vapor deposition of anti-reflection film is impossible at high temperature and the adhesion force of the coating tends to become weak.
Secondly, it can be mentioned that an edge portion is present on the grating and therefore it is difficult to vapor-deposit anti-reflection coating uniformly. This causes the diffraction efficiency to be reduced because as shown in FIG. 14 of the accompanying drawings, the grating shape after the coating has been vapor-deposited does not become a desired shape. To obtain a desired anti-reflection characteristic, it is necessary a multilayer coating construction, and the deformation of the grating shape becomes more remarkable as the number of coating layers is increased and therefore, it is considerably difficult to effect anti-reflection and yet improve the diffraction efficiency.
On the other hand, as opposed to forming the anti-reflection coating as previously described by vapor deposition, there is known a proposition to provide sub-wavelength grating structures used to thereby give an anti-reflection effect, as shown in such literature as J. Opt. Soc. Am A/Vol. 13, No. 5/pp. 988-992/1996 and FIG. 4B of U.S. Pat. No. 5,581,405.
So, the present invention has as its object to solve the above-noted problems and to provide an inexpensive and highly accurate diffractive optical element which has a construction excellent in manufacture and which can greatly reduce the reflection on the surface of a diffraction grating and can maintain high diffraction efficiency.
In order to achieve the above object, a diffractive optical element according to an embodiment of the present invention has a diffraction optical part provided with a phase type diffraction grating, and is characterized in that the diffraction grating of the diffraction optical part has on the surface thereof minute uneven structure of which the period is substantially constant and the period is smaller than a wavelength used.
Also, a diffractive optical element according to the present invention from another point of view has a first diffraction optical part provided with a phase type diffraction grating, and a second diffraction optical part provided with a phase type diffraction grating formed of a material differing from that of the first diffraction optical part, and is characterized in that at least one of the diffraction gratings of the first diffraction optical part and the second diffraction optical part has on the surface thereof minute uneven structure smaller than a wavelength used.